Conventionally, in a fuel injector for an engine, an electro-magnetic valve has a coil that is opened by receiving a supply of electricity to the coil.
A fuel injection control device controls an injector to control an injection of fuel. The fuel injection control device also controls a fuel injection amount and timing by controlling an electricity supply timing of electricity supplied to the coil.
In such a fuel injection control device, a charge voltage for charging a capacitor is set to a target voltage by boosting the electricity supply voltage with a booster circuit and by charging the capacitor at the boosted voltage. Then, at a time of starting the electricity supply to the coil, a switch that connects the capacitor to an upstream side of the coil (hereinafter “discharge switch”) is switched ON to supply a peak current from the capacitor to the coil for a quick opening of the valve in the injector. Thereafter, a constant electric current is supplied to the coil for keeping the injector in a valve-open state until the end timing of the electricity supply time. Further, the fuel injection control device collects, to the above-mentioned capacitor via a diode, the flyback energy (i.e., a counter-electromotive energy) that is generated at the end timing of the electricity supply time for supplying electricity to the coil. Such an arrangement is disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. 2002-303185).
In the conventional fuel injection control device, when an open failure which disables a switch-ON of the discharge switch, the following problems are caused.
Although a discharge current from the capacitor cannot be supplied to the coil of the injector when an open failure is caused in the discharge switch, the valve opening operation of the injector can still be performed by using a constant current circuit that supplies a constant electric current for the coil.
However, even in such an open failure time, the flyback energy generated at the end timing of the electricity supply time for supplying electricity to the coil will be collected to the capacitor via the above-mentioned diode, and electric discharge for discharging electricity from the capacitor will not be performed since the discharge switch suffers the open failure.
Therefore, the charge voltage of the capacitor will rise due to the flyback energy collected from the coil at every drive time of the injector (i.e., more specifically, at the end timing of the electricity supply time for supplying electricity to the coil each time the injector is driven). Thus, the charge voltage of the capacitor will reach an abnormal voltage that leads to the damage of other circuit elements (i.e., other components other than the discharge switch) immediately, which means that the fuel injection control device soon suffers from multi-component failure.
When the multi-component failure is caused by the abnormal rise of the charge voltage, there is no guarantee of a normal drive operation of the injector, which results in the stopping of the vehicle engine and a hindrance to the ability of the vehicle to travel.
Alternatively, for example, to prevent the multi-component failure, the drive of the injector may be intentionally stopped when the charge voltage of the capacitor is detected to be exceeding a predetermined value. However, such a configuration (i.e., a forceful stop of the injector) will yield the same result. That is, when the discharge switch suffers from the open failure, the charge voltage of the capacitor exceeds the above-mentioned predetermined value immediately. As a result, the engine stops and the ability of the vehicle to travel is hindered.
Therefore, when an open failure is caused in the discharge switch of the conventional fuel injection control device, a retreat travel of the vehicle under control of the driver is hindered, which affects the driver's ability to retreat to a safe place.